Series distributed radio frequency (RF) generator for use in wireless power transfer

ABSTRACT

A distributed radio frequency (RF) generator for wireless power transfer for wireless power transfer is described. A distributed RF generator can include an electrically-conductive loop having at least a first end and a second end that are adapted to be electrically coupled to one or more direct current (DC) power sources, where the loop comprises a plurality of segments, each of the plurality of segments comprising: a length of wire and at least one active component, wherein the at least one active component has a first terminal and a second terminal that are electrically coupled to the loop, wherein: a DC voltage exists between the first terminal and the second terminal; a DC current flows into the first terminal and out of the second terminal; an oscillating RF voltage is output across the first terminal and the second terminal; and the at least one active component is synchronized in phase.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national phase application under 35 U.S.C. § 371of International Application No. PCT/US2020/014888 entitled “SERIESDISTRIBUTED RADIO FREQUENCY (RF) GENERATOR FOR USE IN WIRELESS POWERTRANSFER,” filed Jan. 24, 2020, which claims the benefit of and priorityto U.S. Provisional Patent Application No. 62/796,358 entitled “SERIESDISTRIBUTED RADIO FREQUENCY (RF) GENERATOR FOR USE IN WIRELESS POWERTRANSFER,” filed Jan. 24, 2019, the contents of both of which beingincorporated by reference in their entirety herein.

BACKGROUND

Wireless power transfer is the transmission of electrical energy from apower source to an electrical load without the use of man-madeconductors to connect the power source to the electrical load. Awireless power transfer system consists of a transmitter and one or morereceiver devices. The transmitter is connected to a source of power andconverts the power to a time-varying electromagnetic field. The one ormore receiver devices receive the power via the electromagnetic fieldand convert the received power back to an electric current to beutilized by the electrical load.

The recent proliferation of small sensors and the Internet-of-Things(IoT) has introduced a new need for powering a large number of smalldevices within a large, pre-defined area, such as a room, factory, grainsilo, etc. Because wires limit device mobility, and batteries placestrict limitations on device functionality and lifetime, a wirelesspower solution is desirable.

BRIEF SUMMARY OF THE INVENTION

In general, a distributed radio frequency (RF) generator for wirelesspower transfer, and a system and a method thereof, are described forwireless power transfer. A distributed RF generator can include anelectrically-conductive loop having at least a first end and a secondend that are adapted to be electrically coupled to one or more directcurrent (DC) power sources, where the loop comprises a plurality ofsegments, each of the plurality of segments comprising: a length of wireand at least one active component, wherein the at least one activecomponent has a first terminal and a second terminal that areelectrically coupled to the loop, wherein: a DC voltage exists betweenthe first terminal and the second terminal; a DC current flows into thefirst terminal and out of the second terminal; an oscillating RF voltageis output across the first terminal and the second terminal; and the atleast one active component is synchronized in phase.

The distributed RF generator further includes at least one passivesub-segment comprising a length of wire and at least one passivecomponent, the at least one passive component comprising at least onecapacitor; and at least one active sub-segment, the at least one activesub-segment comprising a length of wire and the at least one activecomponent. The at least one passive component can include the at leastone capacitor and at least one RF choke connected in parallel with theat least one capacitor. The at least one capacitor can be preselectedsuch that the loop is series resonant at a pre-determined frequency. Theat least one RF choke can include an inductor and can be preselected tohave a high impedance at the pre-determined frequency so as not tosignificantly affect the resonance of the loop.

Each active component can include a zero voltage switching (ZVS)configuration. Each ZVS configuration can include control circuitry thatautomatically performs ZVS based on a sensed condition within therespective active component; and the sensed condition within therespective active component comprises a sensed RF current. A phasesynchronization of the at least one active component can be performedautomatically in that all of the active component are phase synchronizedto the same RF current flowing in the loop, and each ZVS configurationcomprises a plurality of transistors configured as anelectrically-controllable switch that operates as ZVS amplifier. EachZVS amplifier can be a Class E amplifier.

The DC power source can be one of a plurality of direct current (DC)power sources; and the first end and the second end of theelectrically-conductive loop are adapted to be electrically coupled tothe plurality of DC power sources.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a series distributed RF generator having a number of linearsegments, joined in series, to form a loop, in accordance with anembodiment.

FIG. 2 is a simplified schematic diagram of the RF generator shown inFIG. 1 with a single active component having a Class-E amplifier thatperforms Zero Voltage Switching (ZVS).

FIG. 3 are graphs showing simulated voltage and current waveforms forthe active component having the Class-E ZVS amplifier shown in FIGS. 2and 4 .

FIG. 4 is a simplified block diagram of the active component having aClass-E ZVS amplifier in which a control signal drives anelectrically-controllable switch, S₁, of the amplifier in accordancewith an embodiment.

FIG. 5 shows a distributed RF generator having active components thatinclude the Class E amplifier in accordance with an embodiment withwireless power receivers disposed in a wireless power transfer area ofthe loop of the distributed RF generator.

FIG. 6 is a simplified block diagram for the active component thatincludes a Class-E ZVS amplifier and that uses a resistor, R_(sense), tosense the RF current flowing between the positive and negative terminalsof the active component in accordance with an embodiment.

FIG. 7 is a graph that shows an example of a Current Sense Input signal(sine wave) and a Control Signal Output (square wave) in accordance withan embodiment.

FIGS. 8-11 depict block diagrams of different representative embodimentsof the control circuitry of the active components shown in FIGS. 4 and 6, respectively.

FIG. 12 is a graph which shows the relation between the voltage andcurrent waveforms of the active component, and the relation between thevoltage waveform and the duty cycle of the Control Signal.

FIG. 13 and FIG. 14 are block diagram for a Class-E amplifier withautomatic zero voltage switching (AZVS).

FIG. 15 is a Class-E amplifier rearranged such that a RF generator drawsDC power from the same two terminals it uses to output RF power inaccordance with various embodiments of the present disclosure.

FIG. 16 is a simplified circuit schematic of a resonant magnetic loopantenna driven by distributed RF generators in accordance with variousembodiments of the present disclosure.

FIG. 17 is a photograph showing a physical embodiment of a wirelesspower system in accordance with various embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The present disclosure relates to a series distributed radio frequency(RF) generator for use in wireless power transfer. In some embodiments,wireless power is delivered through a resonant near-field magnetic loopantenna, which fills an entire volume of interest with an oscillatingmagnetic field. In order to simplify installation, it is desirable forthe system to be relatively insensitive to both the exact shape and sizeof the loop antenna, while also maintaining consistent operation undervarying load conditions. Accordingly, various embodiments are describedfor a distributed RF generator design, which can drive a resonantnearfield magnetic loop antenna while simultaneously satisfying theseconditions.

Specific embodiments of the invention will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency. In thefollowing detailed description of embodiments of the invention, numerousspecific details are set forth in order to provide a more thoroughunderstanding of the invention. However, it will be apparent to one ofordinary skill in the art that the invention may be practiced withoutthese specific details. In other instances, well-known features have notbeen described in detail to avoid unnecessarily complicating thedescription.

In the following description, any component described with regard to afigure, in various embodiments of the invention, may be equivalent toone or more like-named components described with regard to any otherfigure. For brevity, at least a portion of these components areimplicitly identified based on various legends. Further, descriptions ofthese components will not be repeated with regard to each figure. Thus,each and every embodiment of the components of each figure isincorporated by reference and assumed to be optionally present withinevery other figure having one or more like-named components.Additionally, in accordance with various embodiments of the invention,any description of the components of a figure is to be interpreted as anoptional embodiment which may be implemented in addition to, inconjunction with, or in place of the embodiments described with regardto a corresponding like-named component in any other figure. In thefigures, black solid collinear dots indicate that additional componentssimilar to the components before and/or after the solid collinear dotsmay optionally exist.

Throughout the application, ordinal numbers (e.g., first, second, third,etc.) may be used as an adjective for an element (i.e., any noun in theapplication). The use of ordinal numbers is not to imply or create anyparticular ordering of the elements nor to limit any element to beingonly a single element unless expressly disclosed, such as by the use ofthe terms “before”, “after”, “single”, and other such terminology.Rather, the use of ordinal numbers is to distinguish between theelements. By way of an example, a first element is distinct from asecond element, and the first element may encompass more than oneelement and succeed (or precede) the second element in an ordering ofelements.

In general, embodiments of the invention provide a method of using adistributed RF generator for wireless power transfer, and a system forwireless power transfer. FIG. 1 shows a series distributed RF generator1 in accordance with a representative embodiment. The distributed RFgenerator 1 includes a number of linear segments, joined in series toform a loop. The two ends of the loop are joined by an RF bypasscapacitor. The RF bypass capacitor is connected across a DC powersupply, V_(DC), through two RF chokes, which block RF current andvoltage from passing to the DC power supply.

Note that it is also possible to provide DC power to the loop from morethan one DC power supply. In such a case, the loop can have two or moreRF bypass capacitors connected in series with the loop. Each RF bypasscapacitor can be connected across a DC power supply through RF chokes,which block RF current and voltage from passing to the DC powersupplies. The DC supplies are connected to the loop so that their DCvoltages add.

In various embodiments, each segment of the loop comprises one activesub-segment and a number of passive sub-segments. Each sub-segmentincludes a length of wire having some inductance per unit length, l. Theinductance of each of these lengths of wire is depicted in FIG. 1 by alumped-element inductor symbol. Each passive sub-segment is connected tothe preceding sub-segment by a passive joint. In accordance with arepresentative embodiment, each passive joint comprises a capacitor inparallel with an RF choke. The capacitors in all of the passive jointsare chosen so that the entire loop is series-resonant at apre-determined frequency. The RF chokes in all of the passive joints canbe selected to have a high impedance at the pre-determined frequency soas to not significantly affect the resonance of the loop, while stillallowing DC current to flow around the loop.

There are two notable frequencies of the distributed RF generator,namely, the aforementioned pre-determined frequency and the drivefrequency. The pre-determined frequency is the self-resonant frequencyof the loop defined as its effective series LRC resonance frequency whenall of the active components are shorted. The drive frequency is set bythe internal oscillator or narrow band-pass filter inside of each activecomponent, as will be described below in more detail. These twofrequencies are not equal to each other. However, they are related toeach other by the ZVS requirement in embodiments in which a ZVSconfiguration is included.

The drive frequency may be chosen such that the drive frequencydetermines a range of self-resonant frequencies for which ZVS ispossible. The actual self-resonant frequency will depend on detuningfrom objects in the environment surrounding the loop, and from the loadcondition. The capacitors in the passive joints can be chosen so thatthe typical self-resonant frequency of the loop will be close to themiddle of the range of self-resonant frequencies which the automatic ZVS(AZVS) configuration can accommodate, as will be described below in moredetail.

In accordance with a representative embodiment, each active sub-segmentmay include a length of wire and an active component. In accordance witha representative embodiment, the active component is an RF generatorhaving first and second terminals. The first and second terminals areused simultaneously as an input for DC power and as an output for RFpower, respectively. The DC power supply, V_(DC), applies a constant DCvoltage between the two ends of the loop which are joined by the RFbypass capacitor. This DC voltage is distributed approximately equallyacross all of the active components in the loop.

The DC power supply also supplies a constant DC current which circulatesaround the loop. This DC current flows through the two terminals of eachof the active components. The product of the DC voltage and the DCcurrent of each active component is equal to the DC power supplied tothat component. Each active component absorbs DC power and converts itinto RF power. This RF power is output by means of an oscillating RFvoltage which is generated across the two terminals of each activecomponent and linearly superimposed upon the DC voltage which appearsacross the same two terminals.

The oscillating RF voltages generated by all of the active componentsadd in series. The RF voltages are synchronized so that they addconstructively. The total series RF voltage generated by all of theactive components drives an RF current which circulates around the loop,and the phase synchronization of the active components is achieved byphase-locking each active component to the phase of this common RFcurrent which is shared by all of the active components on account oftheir series connection.

In general, the RF current circulating around the loop can be used toperform useful work by delivering RF power to any number of RF loads,which may be placed in series with the loop, or may be inductively orradiatively coupled to the loop. In some instances, the activecomponents may be Class E amplifiers with automatic zero-voltageswitching (ZVS), as will be described below in more detail.

In some cases, it may be desirable to suppress the conduction of signalsat harmonic multiples of the drive frequency of the distributed RFgenerator. In such cases, it may be necessary or desirable to place RFfilters in series with the loop which pass signals at DC and the drivefrequency, but which block signals at one or more harmonics of the drivefrequency. Such filters may comprise low-pass filters, tuned resonantfilters, etc. If such filters are present, the series resonant frequencyof the loop should be tuned while including the reactive effects ofthese filters at the drive frequency of the distributed RF generator.

Now, the need for a distributed amplifier is briefly described. Considerthe case of a wireless power system consisting of a single loop withdistributed capacitance shaped in a circular, rectangular, or zig-zagpattern. The loop is driven with a pre-determined RF current amplitudeat a pre-determined RF frequency, and is tuned to behave as a series LRCcircuit which is resonant at the pre-determined frequency.

If the loop were driven at one point by a single RF amplifier, problemsarise when the loop becomes large. Regardless of the shape of the loop,its resistance per unit length and its inductance per unit length eithergrow or approach constant values as the size of the loop grows withoutbound. If the loop were driven by a single RF amplifier at a singledrive point, this implies that the input impedance of the loop at thedrive point grows continually larger as the size of the loop grows andits total length of wire increases.

This means that it will become progressively more difficult to achieve aproper impedance match between the loop and the RF amplifier as the sizeof the loop is increased. Additionally, the high impedance at the drivepoint means that a high RF voltage and high electric field will bepresent at that point and in its vicinity. This electric field has thepotential to cause losses in nearby dielectrics. A high electric fieldalso causes safety concerns due to the possibility of inducing highvoltages on nearby conductors through capacitive coupling, and also dueto absorption of RF power in human tissue via stray electric fields.

These problems may be solved by driving the loop from multiple points,as depicted in FIG. 1 . The system shown in FIG. 1 has the additionaladvantages that all of the RF amplifiers in the loop derive their powerfrom the loop, and that they lock their phase using the common RFcurrent. In other words, no additional wiring is needed to drive theamplifiers other than the wire of the loop itself.

Next, the principals of operation of a Class-E ZVS amplifier aredescribed. FIG. 2 is a simplified schematic diagram of the RF generatorshown in FIG. 1 with a single active component having a Class-Eamplifier that performs Zero Voltage Switching (ZVS) in accordance witha representative embodiment. Power is provided to the amplifier from theDC power supply, V_(DC). The current, I, flowing around the loop hasboth a DC and an RF component. The RF Chokes, labeled RFC, block the RFcomponent of the current, I, but pass the DC component with negligibleresistance.

The active component includes an electrically-controllable switch, S₁,in parallel with a capacitor, C₁, along with control circuitry forswitching S₁ on and off (not shown). When the amplifier 20 is in asteady state, the switch S₁ is switched on and off with a constantperiod, T, and duty cycle, δ. When S₁ is on, the current, I flowsthrough S₁ and the voltage, V_(D), across the active component is zero.When S₁ is off, the current, I, flows into capacitor C₁, causing it tocharge. If the DC component of the current is sufficiently small, thevoltage, V_(D), across capacitor C₁, will be approximately sinusoidal,and will return to zero after a time period shorter than the switchingperiod, T.

To maximize efficiency, it is optimal to run the amplifier in a modecalled Zero Voltage Switching (ZVS). In this mode, the timing of theswitch, S₁, is chosen such that it is switched on at the same momentwhen the voltage of capacitor, C₁, reaches zero. This ensures that thecapacitor, C₁, does not have any stored electrical energy when theswitch, S₁, is turned on. If the ZVS condition is not met, then anyelectrical energy stored in capacitor, C₁, will be dissipated as heat inthe resistance of switch, S₁, at the instant when the switch, S₁, isturned on.

Assuming that the ZVS condition is met, and that the switch, S₁, isclose to ideal, then the switch, S₁, will dissipate negligible power.The switch, S₁, has a positive DC voltage across it, equal to theaverage of the voltage, V_(D), over one period. The active componentmust therefore absorb a DC power equal to the product of this DC voltageand the DC component of the current, I.

Because the active component dissipates negligible power in the ZVScondition, nearly all of the DC power absorbed by the active componentis converted into RF power. This RF power is delivered to the RF load,R_(LOAD), and maintains the RF current circulating around the loop.

FIG. 3 is a graph showing simulated voltage and current waveforms forthe active component of the Class-E ZVS amplifier shown in FIGS. 2 and 4. The top plot shows the RF component of the current waveform (solidline) and the DC component of the current waveform (dashed line) flowingthrough the inductor, L0, of FIG. 2 , or into the positive terminal andout of the negative terminal of the active component depicted in FIG. 4. The bottom plot shows the voltage waveform across switch S1 (solidline), and its average value over one cycle (dot-dash line). The averagevalue of the voltage waveform is equal to the DC voltage across the twoterminals of the active component. Also depicted in the bottom plot ofFIG. 3 is the control signal waveform for the electrically-controllableswitch, S1. This control waveform is a binary logic control signal. Whenthe control signal is logic high, S1 is on. When the control signal islogic low, S1 is off. The switch S1 is turned on at the instant in timewhen the control signal undergoes an low-to-high transition. Note thatthis transition is synchronous with the zero-crossing of the voltagewaveform of the active component, which means that this Class-Eamplifier is operating in Zero Voltage Switching (ZVS) mode.

FIG. 4 shows a simplified block diagram for the active component 40 thatincludes a Class-E amplifier with AZVS in accordance with arepresentative embodiment. This active, two terminal, componentrepresents the S₁, C₁ combination shown in the simplified schematic ofFIG. 2 , along with the additional circuitry which generates the controlsignal for the electrically-controllable switch, S₁.

The control circuitry may derive its DC power from the positive andnegative terminals of the active component. It may use some combinationof filtering, rectification, regulation, switching and/or DC-to-DCconversion to generate a constant DC voltage from the time-varyingvoltage, V_(D), present across these two terminals. Alternatively, itmay use some combination of filtering, rectification, regulation,switching and/or DC-to-DC conversion to generate a constant DC voltagefrom the RF voltage induced in the secondary inductor, L₂, in thetransformer formed by the coupled inductors, L₁ and L₂.

In accordance with an embodiment, the control circuitry generates asquare wave with period, T, duty cycle, δ, and phase, φ. The period, T,is set by an internal frequency reference within the control circuitry,such as a quartz crystal or MEMs resonator. The control circuitry variesthe duty cycle, δ, based on feedback derived from sensing the voltagewaveform, V_(D). If the voltage waveform, V_(D), reaches zero before theinstant in time when the switch, S₁, is turned on, the control circuitryincreases the duty cycle, δ, causing the switch, S₁, to turn on soonerin the cycle. If the voltage waveform, V_(D), does not yet reach zero atthe instant in time when the switch, S₁, turns on, the control circuitrydecreases the duty cycle, δ, causing the switch, S₁, to turn on later inthe cycle. The control circuitry continually monitors the voltagewaveform, V_(D), and adjusts the duty cycle, δ, accordingly in order tomaintain the ZVS condition. The timescale of this feedback mechanism ischosen to be much slower than the switching period, T.

By the mechanism described above, the active component is able toautomatically maintain the ZVS condition over a wide range of tuning andload conditions of the Class-E amplifier. This allows efficient ZVSoperation to be maintained without the need for precise tuning orimpedance matching.

Note that the AZVS mechanism described above may also be applied to anRF amplifier consisting of a single Class-E amplifier, as depicted inFIG. 13 . A square wave generator, such as a variable duty-cycle squarewave generator 203, can generate a square wave with a frequency andphase determined by the input RF signal. The input signal may be a sinewave, a square wave, or some other waveform. The output of the squarewave generator is a square wave with a duty cycle determined by afeedback signal from a ZVS timing offset detector 206.

The square wave generator 203 may also incorporate a duty-cycledependent phase shift. The switching of S1 converts the DC power fromthe input DC power terminal, VDC, into an RF power, output to a load,depicted as an effective series resistance, Rload. In practice, a loadmay be coupled in series with the inductor, L0, in parallel withcapacitor, C0, inductively coupled with inductor, L0, or coupled by anyother mechanism which transfers RF power from the tank circuit,consisting of L0 and C0, to the RF load. The feedback from the ZVStiming offset detector 206 automatically controls the duty cycle of thedrive signal for switch, S1, to ensure that the ZVS condition issatisfied for a wide range of values of the load resistance and tuningof the L0, C0 tank circuit, therefore allowing efficient operationwithout the need for precise tuning and impedance matching of the outputnetwork of the Class-E amplifier.

Next, the series combination of active components is described. Inaccordance with an embodiment, the active component shown in FIG. 4 ,which can be used as the active components shown in FIG. 1 , derives itspower from the same two terminals which it uses to output RF power. Ifmultiple active components are connected in series in a loop, as shownin FIG. 1 , a single DC power supply can power all of the activecomponents simultaneously through the loop itself, without the need foradditional wiring. The RF power of all of the active components willadd, so long as the timing is arranged so that all of the activecomponents are switching in phase.

FIG. 5 shows a distributed RF generator having active components thatinclude the Class E amplifier in accordance with an embodiment withwireless power receivers disposed in a wireless power transfer area ofthe loop of the distributed RF generator. The phase synchronization ofthe active components may be established by arranging the activecomponents to lock their phases to a single common source. Because allof the active components are in series, they all share the same RFcurrent. If each active component maintains a constant phase relationwith respect to this RF current, then they all maintain a constant phaserelation with respect to each other. Therefore, phase synchronization ofthe active components can be established by locking the phase of eachactive component to have a constant offset relative to the phase of theRF current flowing through it.

FIG. 4 illustrates one means by which this may be accomplished. Thetransformer, comprising inductors L₁ and L₂, picks up an induced RFvoltage, V_(ind), which is linearly proportional to the RF currentflowing through the active component. This signal is fed to the controlcircuitry, and is used by the control circuitry to set the frequency andthe phase of the square wave which it generates as the control signal.This may be accomplished passively using for example, a narrow band-passfilter, or actively using for example, a phase-locked loop or aninjection-locked oscillator.

FIG. 6 is a simplified block diagram for the active component thatincludes a Class-E ZVS amplifier and that uses a resistor, R_(sense), tosense the RF current flowing between the positive and negative terminalsof the active component in accordance with an embodiment. FIG. 6 alsoillustrates an alternative method for locking the phases of the activecomponents relative to each other. Instead of using the transformershown in FIG. 4 to sense the RF current, the active component depictedin FIG. 6 uses a series resistor, R_(sense), to sense the RF currentflowing between the two terminals of the active component. Note that thevoltage across the resistor, R_(sense), is shifted in phase by 90°relative to the voltage, V_(ind), induced in inductor L₂ in FIG. 4 .Therefore, additional circuitry, depicted by the block labeled “90°phase shift” in FIG. 6 , is needed to accommodate this 90° phasedifference. This additional 90° phase shift may be incorporated into thebehavior of the control circuitry itself.

Because all of the active components maintain a constant phase withrespect to each other, this phase-lock mechanism will also ensure thatthey all operate at the same frequency, regardless of any variance intheir internal frequency references.

Now, the relation between phase and duty cycles is discussed. FIG. 7shows an example of a Current Sense Input signal (sine wave) and aControl Signal Output (square wave). FIG. 7 also graphically defines thephase angle between the Current Sense Input (sine wave) and the ControlOutput (square wave). Let the relative phase angle between the two wavesbe defined as the phase difference between the complex phasors of thefundamental components of their respective Fourier series. Thisdefinition of phase angle implies that the sine wave and the square waveare in phase when the center of the square pulse (defined as halfwaybetween the rising and the falling edge) is synchronous with the peak ofthe sine wave.

Under light load and under typical tuning conditions, the duty cycle ofthe Control Signal output will be approximately 50% and the phase shiftbetween the Control Signal and the RF Current Sense input will beapproximately zero. Under heavy load, or in cases where the resonantloop is detuned, the duty cycle of the Control Signal output will needto differ from 50% in order to achieve ZVS. If the active components areconnected in a series chain, each active component will adjust its dutycycle independently in order to achieve the ZVS condition.

If the phase shift between the Current Sense Input and the ControlSignal Output of each control circuit is independent of the duty cycle,then the chain of amplifiers develops an instability in which the totalseries DC voltage around the loop is not equally shared by all of theactive components. In order to eliminate this instability, it isnecessary for each control circuit to introduce a duty-cycle-dependentphase shift, Δφ, between the Current Sense input and the Control Signaloutput. One example of a duty-cycle-dependent phase shift whicheliminates this instability is given by the following equation:

$\begin{matrix}{{\Delta_{\varnothing_{i}} = {\pi\;{k\left( {\delta_{i} - \frac{1}{2}} \right)}}},} & \left( {{eq}.\mspace{14mu} 1} \right)\end{matrix}$where Δ_(ϕ) _(i) and δ_(i) are the phase shift and the duty cycle of theith active component, respectively, and the coefficient, k, is adimensionless number greater than 0.The system becomes more stable for larger values of k. Note that if k=1,then the falling edge of the Control Signal Output remains synchronouswith the negative-slope zero-crossing of the Current Sense Input,regardless of the duty cycle, δ_(i).

The electrically-controllable switch, S1, depicted in FIGS. 2, 4 and 6 ,may be implemented using, for example, a MOSFET, a BJT, or anyelectrically-controllable switch with a switching speed significantlyfaster than the pre-determined operating frequency of the system.

FIGS. 8-11 depict block diagrams of different representative embodimentof the control circuitry of the active components 40 and 60 shown inFIGS. 4 and 6 , respectively. The control circuitries 80 and 90 shown inFIGS. 8 and 9 have AZVS configurations, whereas the control circuitry100 shown in FIG. 10 does not. These active, two terminal, componentsrepresents the S₁, C₁ combination shown in the simplified schematic ofFIG. 2 , along with the additional circuitry which generates the controlsignal for the electrically-controllable switch, S₁.

The control circuitries 80-100 may derive their DC power from thepositive and negative terminals of the active component. It may use somecombination of filtering, rectification, regulation, switching and/orDC-to-DC conversion to generate a constant DC voltage from thetime-varying voltage, V_(D), present across these two terminals.Alternatively, it may use some combination of filtering, rectification,regulation, switching and/or DC-to-DC conversion to generate a constantDC voltage from the RF voltage induced in the secondary inductor, L₂, inthe transformer formed by the coupled inductors, L₁ and L₂ (FIG. 9 ).FIGS. 8 and 10 depict this sensing device more generally by its functionas “RF current sensing.”

In each of the embodiments shown in FIGS. 8-10 , an RF-to-DC converterand/or filter block 101 (FIGS. 8 and 10 ), 102 (FIG. 9 ) of the controlcircuitries 80-100 provides power to an oscillator circuit whichgenerates a square wave with period, T, duty cycle, δ, and phase, φ. Inthe embodiment shown in FIG. 11 , the RF Current Sensing device 104,Narrow Band-Pass Filter 112, and Impedance Matching 113 produce a drivesignal which in some cases may be a sine wave with period T, with a DCoffset provided by the DC Bias Generator 111. The amplitude and DCoffset of this sine wave are chosen such that the electricallycontrolled switch, S1, is driven in its switching mode. The period, T,is set by an internal frequency reference within the control circuitry,such as a quartz crystal, in the phase-locked oscillator 103. Thecontrol circuitry 80-110 sets the phase of the control signal based onthe phase of the voltage waveform output from the current sensing device104. If the control circuitries implement AZVS (FIGS. 8 and 9 , then ifthe voltage waveform, V_(D), reaches zero before the instant in timewhen the switch, S₁, is turned on, this is detected by ZVS timing offsetdetector 105, the output of which is used by a duty-cycle shifter andphase shifter block 106 (FIGS. 8 and 9 ) to increase the duty cycle, δ,causing the switch, S₁, to turn on sooner in the cycle. If the voltagewaveform, V_(D), does not yet reach zero at the instant in time when theswitch, S₁, turns on, this is detected by ZVS timing offset detector105, the output of which is used by the duty-cycle shifter and phaseshifter block 106 to decrease the duty cycle, δ, causing the switch, S₁,to turn on later in the cycle. The control circuitry continuallymonitors the voltage waveform, V_(D), and adjusts the duty cycle, δ,accordingly in order to maintain the ZVS condition. The timescale ofthis feedback mechanism is chosen to be much slower than the switchingperiod, T.

The switch S1 of the active component can be an amplifier comprising aplurality of transistors, which are typically MOSFETs. The amplifier caninclude a Class E amplifier in some embodiments. While it is known touse ZVS configurations in Class E amplifiers, it is not known to useAZVS configurations in Class E amplifiers. By the AZVS mechanismdescribed above, the active component is able to automatically maintainthe ZVS condition over a wide range of tuning and load conditions of theClass-E amplifier. This allows efficient ZVS operation to be maintainedwithout the need for precise tuning or impedance matching.

As depicted in FIG. 10 , however, the control circuitry can perform itsfunctions without implementing AZVS, but this may result in some energybeing dissipated in the switch, S1, if the timing of switching is suchthat the switch is placed in the on state before V_(D) has fullyreturned to zero volts. Thus, using the AZVS mechanism improvesperformance and efficiency.

With reference to FIG. 11 , the active component 110 uses a passivefilter 112 instead of an active oscillator to drive theelectrically-controllable switch S1. This filter 112 is a narrowband-pass filter that has a maximum RF transmission at the desired drivefrequency of the system. The input voltage for the filter 112 comes fromthe RF current sensing device 104, which senses the RF current in theloop and outputs a voltage proportional to the RF current passingthrough the active component 110. As indicated above, examples of suchcomponents may be an RF transformer or a series resistor, which arepassive components requiring no external power. The output of the filter112 is impedance-matched to the input of the electrically-controllableswitch S1 by an impedance matching circuit 113. The impedance matchingis chosen such that the voltage and power drive requirements for theelectrically-controllable switch S1 are satisfied for somepre-determined range of RF current levels.

The phase shift between the input and the output of the band pass filter112 is chosen so that the drive signal at the input of theelectrically-controllable switch S1 will have the proper timing tosatisfy the ZVS requirement for a pre-determined tuning and loading ofthe loop. In addition to the signal generated by the RF current sensingdevice 104, the band-pass filter 112, and the impedance matching circuit113, an additional DC bias voltage from the DC bias generator 111 isadded to the voltage input to the electrically-controllable switch S1through a voltage-summing mechanism 114. Examples of voltage summingmechanisms for adding DC and RF voltages include networks of resistors,capacitors, inductors, and/or transformers.

The DC bias generator 111 serves two purposes. First, it allows thesystem to begin oscillation upon start-up. When power is first applied,there is no RF current. Therefore, there is no RF input to the band-passfilter 112 and consequently no RF drive signal to theelectrically-controllable switch S1. The DC bias generator 111 biasesthe electrically-controllable switch S1 to an intermediate state betweenfully on and fully off. When the electrically-controllable switch S1 isin this state, it can behave as a linear amplifier. Any fluctuation inthe output of this linear amplifier causes a proportional fluctuation inthe current in the loop. The RF current sensing device 104 picks upthese fluctuations and sends them, through the band-pass filter 112, theimpedance matching circuit 113, and summing network 114, to the input ofthe electrically-controllable switch S1. The system of the activecomponent 110, therefore, forms a closed feedback loop. This feedbackloop provides positive feedback, causing the system to oscillate at afrequency somewhere within the bandwidth of the narrow band-pass filter.Small initial fluctuations are amplified into a large oscillation. Theoscillation grows exponentially until the electrically-controllableswitch is saturated and is driven into its switching mode, at whichpoint it becomes an efficient, Class-E amplifier.

The second purpose that the DC bias generator 111 serves is that itallows the duty cycle of the electrically-controllable switch S1 to beset to a pre-determined value, dependent upon the expected tuning,power, and load conditions of the loop. Note that this system naturallycauses all of the active components to switch in phase. The phase ofeach drive signal is determined by the phase shift of its internalnarrow band-pass filter 112 and the phase of the RF current circulatingin the loop. The RF current is shared by all of the active components.Therefore, if the narrow band-pass filters 112 all have phase shiftswhich are approximately the same, the phase of the drive waveforms forall of the electrically-controllable switches S1 of the loop will beapproximately the same, and the RF voltage waveforms of all of theactive components will add constructively.

FIG. 12 shows simulated voltage and current waveforms for a singleactive component. The upper plot shows the RF current (solid curve) andthe DC current (dashed curve) flowing through the two terminals of theactive component. The lower plot shows the voltage across the twoterminals of the active component for three different values of the dutycycle, delta. In this example, the coefficient, k, is set to 0.8.

In this example, when the duty cycle of the Control Signal Output is 55%(dashed curve), the voltage has not yet reached zero at the point intime when the switch, S1, is turned on. It can be seen from the plotthat the voltage suddenly drops to zero when the switch is turned on,implying that the electrical energy stored in capacitor, C1, isdissipated as heat.

When the duty cycle of the Control Signal Output is 50% (solid curve),the voltage is exactly zero at the point in time when the switch, S1, isturned on. Therefore, no heat is dissipated in the switch, S1, at theinstant of switching, since the capacitor, C1, stores no energy at thatinstant of time.

When the duty cycle of the Control Signal Output is 45% (dotted curve),the voltage has already passed zero, and has become negative at thepoint in time when the switch, S1, is turned on. It can be seen from theplot that the voltage suddenly rises to zero when the switch is turnedon, implying that the electrical energy stored in capacitor, C1, isdissipated as heat.

The ZVS Timing Offset Detector 105, shown in the block diagram depictedin FIGS. 8 and 9 detects the timing offset between the zero-crossing ofthe voltage waveform and the turn-on time of switch, S1. From thisdetected offset, it generates the feedback signal which adjusts the dutycycle and phase of the Control Signal Output in order to automaticallymaintain the ZVS condition.

Note that in some embodiments, the switch, S1, will be implemented usinga MOSFET, which contains an internal body diode. The internal body diodewill prevent the voltage across the two terminals of the activecomponent from becoming any more negative than the forward voltage dropof this diode. Therefore, in some embodiments, the third waveform(dotted line) shown in FIG. 12 will be clipped at some negative voltage.This fact will not impact the ability of the ZVS Timing Offset Detectorto detect the timing offset between the zero-crossing of the voltagewaveform and the turn-on time of switch, S1.

Also note that the efficiency of the amplifier will be high so long asthe ZVS condition is approximately satisfied, even if it is not exactlysatisfied. The energy stored in the capacitor, C1, is quadratic withrespect to its voltage. Therefore, the energy lost per cycle has a soft(i.e. quadratic) minimum at the optimal duty cycle for ZVS, meaning thatsmall offsets in the duty cycle relative to the optimal duty cycle willnot substantially degrade the efficiency of the system.

Because the AZVS system can tolerate small deviations from the optimalduty cycle without experiencing substantial degradation in itsefficiency, it is possible to use a ZVS Timing Offset Detector whichdetects the time when the voltage waveform crosses a threshold voltageslightly above or below zero, rather than the time when the voltagewaveform crosses zero. Such a detector may be more practical toimplement, and would yield acceptable performance so long as thethreshold voltage is small relative to the peak voltage. If necessary, asmall time offset could be added to the ZVS Timing Offset Detector basedon the slope of the voltage waveform and the value of the voltagethreshold in order to correct for the small timing error introduced bythe non-zero voltage threshold. Such an offset will make a ZVS TimingOffset Detector with a non-zero voltage threshold better approximate thebehavior of an ideal detector with a voltage threshold of exactly zero.

The ZVS Timing Offset Detector may be implemented in several ways. Threepossible implementations are described as follows. In oneimplementation, the ZVS Timing Offset Detector can be made to sense thetime when the voltage across S1 passes a certain threshold which isgreater than zero. Because this threshold-crossing will be delayed ashort time after the rising edge of the Control Signal due to theturn-on time of switch, S1, the threshold crossing will occur after therising edge of the Control Signal whenever the duty cycle is too long.Alternatively, when the duty cycle is too short, the voltage across S1will already be below the threshold before the rising edge of theControl Signal. The ZVS Timing Offset Detector can therefore use thetime order of the two events (i.e. rising edge of the Control Signal andthreshold crossing of the voltage across S1) as a binary signal todetermine whether the duty cycle should be either increased or decreasedin order to achieve ZVS. In a second implementation, the ZVS TimingOffset Detector can be made to produce a binary signal, which has onelogical value if the voltage across S1 is above a threshold at aninstant of time equal to, or slightly before, the point in time when S1turns on, and the opposite logical value if the voltage across S1 isbelow that threshold at the instant of time equal to, or slightlybefore, the point in time when S1 turns on.

This binary signal can be generated by making a comparison between thevoltage across switch S1 and a threshold voltage, and storing thatbinary value in a digital latch, clocked by a signal synchronous with,or slightly preceding, the turn-on of S1. In some cases, a slight delaymay be needed to satisfy the set-up and hold times of the latch. If thatis the case, a slight delay may be added between the rising edge of theControl Signal and the turn-on of switch, S1, therefore ensuring thatthe logical comparison between the voltage across S1 and the thresholdvoltage can be properly measured before the voltage across S1 drops tozero. In a third implementation, if a linear feedback control signal isdesired, the ZVS Timing Offset Detector can use a clockedsample-and-hold circuit to sample the value of the voltage across switchS1 at the instant when S1 turns on. A slight delay may be added betweenthe rising edge of the Control Signal and the turn-on of S1 if necessaryto satisfy the set-up and hold times of the sample-and-hold circuit.

Another embodiment for automatic zero-voltage switching (AZVS) is shownin FIG. 14 . A comparator and latch can detect whether the drain voltageof Q1 is above or below a reference voltage, VREF, at the instant whenQ1 turns on. The output of the latch is low-pass-filtered and used as afeedback signal to control the duty cycle of the gate drive waveform.The feedback shifts the time at which Q1 turns on until the drainvoltage of Q1 is equal to VREF at the rising edge of the gate drivesignal. If VREF is very close to zero, then this feedback loopautomatically maintains the amplifier in a state of ZVS operation. Assuch, the reference voltage, VREF, can set to be as close to 0V aspossible, while still being larger than the amplitude of any drainvoltage ringing caused by parasitic inductance. The feedback networkvaries the duty cycle of the gate drive to ensure that ZVS conditionsare always met.

The AZVS amplifier described herein provides efficient operation to bemaintained under varying load conditions, and in the presence ofdetuning due to variations in a variable shape of the loop. However,another problem arises for loops of varying size. As the size of theloop grows, its inductance increases. The total series capacitance, C0,must therefore decrease. If the capacitor, C1, remains fixed, then Kmust decrease. Eventually, the AZVS amplifier will no longer be capableof accommodating a desired tuning range, Δf₀. It is possible to reduceC1 with increasing loop size, but that is undesirable two reasons.First, in order for the RF current amplitude to be held constant, the DCsupply voltage would need to vary inversely with C1, requiring the useof components with a very high voltage rating as the loop size becomeslarge. Second, if the loop is driven from one point, the RF inputimpedance at the drive point grows continually larger as the size of theloop grows.

A high impedance at the drive point means that a high RF voltage andhigh electric field will be present at that point and in its vicinity.This electric field has the potential not only to cause losses in nearbydielectrics, but also safety concerns due to the possible absorption ofRF power in human tissue via stray electric fields. Both of theseproblems are solved by driving the loop from multiple points withmultiple, synchronized RF generators. Each RF generator requires asource of DC power. While it is possible to run a separate power cableto each generator, this would vastly increase the amount of wiring. Itis therefore desirable to power the generators without any additionalwiring other than the wire of the loop itself. This is achieved byadding RF chokes in parallel with the distributed capacitors of theloop, which allow the loop to support both a DC and an RF currentsimultaneously. Each RF generator takes in DC power and outputs RF powerthrough the same two terminals.

FIG. 15 shows a Class-E amplifier modified in order to accomplish theforegoing. Specifically, FIG. 15 shows a Class-E amplifier, rearrangedso that the RF generator draws its DC power from the same two terminalsit uses to output RF power. The gate drive circuit, G₁, uses adrain-to-source voltage of Q₁ as a source of power in addition to usingit as an input signal to the AZVS feedback loop. G₁ also contains anadditional input connected to a current-sensing transformer, which isused to lock the phase of an internal oscillator to the phase of the RFcurrent circulating around the loop. This phase-locking is necessary tolock the phases of multiple RF generators when they are connectedtogether in series.

FIG. 16 shows how multiple RF generators, like the one in FIG. 15 , maybe connected in series and distributed around the loop. Morespecifically, FIG. 16 shows a simplified schematic of a resonantmagnetic loop antenna driven by distributed RF generators. In general,the loop consists of N_(a) active RF generators, and N_(p) passivejoints, connected by lengths of wire. Each passive joint consists of acapacitor in parallel with an RF choke. The RF chokes are chosen to havea high impedance at the drive frequency so as to not significantlyaffect the resonance of the loop, but still allow DC current to flowaround the loop. The RF generators use the same two terminals as both aDC power input and an RF power output.

For the distributed version of the Class-E amplifier, the value of theconstant, K, is given by:

$\begin{matrix}{{K = {\frac{\chi_{1}}{\chi_{1} + \chi_{0}} = {\frac{1}{1 + \frac{\chi_{0}}{\chi_{1}}} = \frac{1}{1 + \frac{N_{p}\chi_{0}^{({{sing}.})}}{N_{a}\chi_{1}^{({{sing}.})}}}}}},} & \left( {{eq}.\mspace{14mu} 2} \right)\end{matrix}$where χ₀ is the total series reactance of all of the passive joints, χ₁is the total series reactance of all of the capacitors of the activejoints, χ₀ ^((sing.)) is the reactance of a single passive joint, and χ₁^((sing.)) is the reactance of the capacitor of a single active joint.If the ratio, N_(p)χ₀ ^((sing.))/N_(a)χ₁ ^((sing.)), is held fixed, theneq. 2 implies that K is independent of the size of the loop. Therefore,unlike the case of a loop containing a single RF generator and singledrive point, the distributed RF generator can accommodate a tuning rangewhich is independent of the size of the loop without any changes tocomponent values.

Phase-Locking. In order to properly drive the loop, all of thedistributed RF generators depicted in FIG. 16 must be synchronized.While this may be achieved by connecting all of the amplifiers to acommon local oscillator via a star configuration, this solution isundesirable, since it would require a separate cable for each RFgenerator, vastly increasing the amount of wiring. It is thereforedesirable to arrange the RF generators so that they lock in phase witheach other without any additional wiring other than the wire of the loopitself. According to various embodiments, phase-locking may be achievedby locking each RF generator to the phase of some RF signal which theyall share in common. Because all of the RF generators are connected inseries, the common RF current shared by all of the RF generatorssuggests itself as a natural choice for this common signal.

FIG. 15 shows how phase-locking can be accomplished. A current-sensingtransformer picks up an induced voltage proportional to the RF currentflowing around the loop. The phase of an internal oscillator circuit isthen locked to the phase of this induced voltage. The phase-lockedoscillator is used to generate the gate-drive waveform. Assuming thatall of the RF generators are identical, this guarantees that thegate-drive waveform of each RF generator has the same phase relative tothe phase of the RF current, which ensures that all of the RF generatorsare in phase with respect to each other.

FIG. 17 is a photograph showing a physical embodiment of the schematicof FIG. 15 . Specifically, FIG. 17 shows a photograph of a wirelesspower system having a distributed RF generator (e.g., three RFgenerators) powering and coupled to three regulated wireless loads (orwireless receivers). Each receiver delivers a regulated 5 W of power toan LED load. The DC input power was 34 W (0.6 A at 57V). The DC-to-DCefficiency was 44% when the loads were present. The system drew 13 W ofDC power when the loads were absent.

It should be noted that representative or illustrative embodiments havebeen described herein for the purpose of demonstrating the inventiveprinciples and concepts. As will be understood by persons of skill inthe art in view of the description provided herein, many modificationsmay be made to the embodiments described herein without deviating fromthe scope of the invention. For example, while the inventive principlesand concepts have been described primarily with reference to particularconfigurations of active components with particular configurations ofamplifiers, the inventive principles and concepts are equally applicableto other configurations that accomplish the goals described herein, aswill be understood by those of skill in the art in view of thedescription provided herein. For example, other classes of amplifiersand/or amplifiers that do not incorporate ZVS principles may be used toachieve the distributed RF generator configuration shown in FIG. 1 .Many other modifications may be made to the embodiments described hereinwithout deviating from the inventive principles and concepts, and allsuch modifications are within the scope of the invention, as will beunderstood by those of skill in the art.

Clause 1. A distributed radio frequency (RF) generator for wirelesspower transfer, comprising: an electrically-conductive loop having atleast a first end and a second end that are adapted to be electricallycoupled to a direct current (DC) power source, wherein the loopcomprises a plurality of segments, each of the plurality of segmentscomprising: a length of wire and at least one active component, whereinthe at least one active component has a first terminal and a secondterminal that are electrically coupled to the loop, wherein: a DCvoltage exists between the first terminal and the second terminal; a DCcurrent flows into the first terminal and out of the second terminal; anoscillating RF voltage is output across the first terminal and thesecond terminal; and the at least one active component is synchronizedin phase.

Clause 2. The distributed RF generator of clause 1, wherein thedistributed RF generator further comprises: at least one passivesub-segment comprising a length of wire and at least one passivecomponent, the at least one passive component comprising at least onecapacitor; and at least one active sub-segment, the at least one activesub-segment comprising a length of wire and the at least one activecomponent.

Clause 3. The distributed RF generator of clauses 1-2, wherein the atleast one passive component comprises the at least one capacitor and atleast one RF choke connected in parallel with the at least onecapacitor.

Clause 4. The distributed RF generator of clauses 1-3, wherein the atleast one capacitor is preselected such that the loop is series resonantat a pre-determined frequency.

Clause 5. The distributed RF generator of clauses 1-4, wherein: the atleast one RF choke includes an inductor; and the at least one RF chokeis preselected to have a high impedance at the pre-determined frequencyso as not to significantly affect the resonance of the loop.

Clause 6. The distributed RF generator of clauses 1-5, wherein eachactive component has a zero voltage switching (ZVS) configuration.

Clause 7. The distributed RF generator of clauses 1-6, wherein: each ZVSconfiguration comprises control circuitry that automatically performsZVS based on a sensed condition within the respective active component;and the sensed condition within the respective active componentcomprises a sensed RF current.

Clause 8. The distributed RF generator of clauses 1-7, wherein: a phasesynchronization of the at least one active component is performedautomatically in that all of the active component are phase synchronizedto the same RF current flowing in the loop; and each ZVS configurationcomprises a plurality of transistors configured as anelectrically-controllable switch that operates as ZVS amplifier.

Clause 9. The distributed RF generator of clauses 1-8, wherein each ZVSamplifier is a Class E amplifier.

Clause 10. The distributed RF generator of clauses 1-9, wherein: the DCpower source is one of a plurality of direct current (DC) power sources;and the first end and the second end of the electrically-conductive loopare adapted to be electrically coupled to the plurality of DC powersources.

Clause 11. A system for wireless power transfer, comprising: a directcurrent (DC) power source; and a distributed radio frequency (RF)generator for wireless power transfer, comprising: anelectrically-conductive loop having at least a first end and a secondend that are adapted to be electrically coupled to the DC power source,wherein the loop comprises a plurality of segments, each of theplurality of segments comprising: a length of wire and at least oneactive component, wherein the at least one active component has a firstterminal and a second terminal that are electrically coupled to theloop, wherein: a DC voltage exists between the first terminal and thesecond terminal; a DC current flows into the first terminal and out ofthe second terminal; an oscillating RF voltage is output across thefirst terminal and the second terminal; and the at least one activecomponent is synchronized in phase.

Clause 12. The system of clause 11, wherein: the DC power source is oneof a plurality of direct current (DC) power sources; and the first endand the second end of the electrically-conductive loop are adapted to beelectrically coupled to the plurality of DC power sources. The systemcan further include the components of the distributed RF generator ofclauses 1-10.

Clause 13. A method for wireless power transfer, comprising: providing adirect current (DC) power source; and providing a distributedradiofrequency (RF) generator comprising an electrically-conductive loophaving at least first and second ends that are electrically coupled tothe DC power source, wherein the loop comprises a plurality of segments,each of the plurality of segments comprising: a length of wire and atleast one active component, wherein the at least one active componenthas a first terminal and a second terminal that are electrically coupledto the loop, wherein: a DC voltage exists between the first terminal andthe second terminal; a DC current flows into the first terminal and outof the second terminal; an oscillating RF voltage is output across thefirst terminal and the second terminal; and the at least one activecomponent is synchronized in phase.

Clause 14. The method of clause 13, wherein the distributed RF generatorfurther comprises: at least one passive sub-segment comprising a lengthof wire and at least one passive component, the at least one passivecomponent comprising at least one capacitor; and at least one activesub-segment, the at least one active sub-segment comprising a length ofwire and the at least one active component.

Clause 15. The method of clauses 13-14, wherein: the DC power source isone of a plurality of direct current (DC) power sources; and the firstend and the second end of the electrically-conductive loop are adaptedto be electrically coupled to the plurality of DC power sources. Themethod can further include providing the components of the distributedRF generator of clauses 1-10 or the system of clauses 11-12.

The invention claimed is:
 1. A distributed radio frequency (RF)generator for wireless power transfer, comprising: anelectrically-conductive loop having at least a first end and a secondend that are adapted to be electrically coupled to a direct current (DC)power source, wherein the loop comprises a plurality of segments, eachof the plurality of segments comprising: a length of wire and at leastone active component, wherein the at least one active component has afirst terminal and a second terminal that are electrically coupled tothe loop, wherein: a DC voltage exists between the first terminal andthe second terminal; a DC current flows into the first terminal and outof the second terminal; an oscillating RF voltage is output across thefirst terminal and the second terminal; and the at least one activecomponent is synchronized in phase.
 2. The distributed RF generator ofclaim 1, wherein the distributed RF generator further comprises: atleast one passive sub-segment comprising a length of wire and at leastone passive component, the at least one passive component comprising atleast one capacitor; and at least one active sub-segment, the at leastone active sub-segment comprising a length of wire and the at least oneactive component.
 3. The distributed RF generator of claim 2, whereinthe at least one passive component comprises the at least one capacitorand at least one RF choke connected in parallel with the at least onecapacitor.
 4. The distributed RF generator of claim 3, wherein the atleast one capacitor is preselected such that the loop is series resonantat a pre-determined frequency.
 5. The distributed RF generator of claim4, wherein: the at least one RF choke includes an inductor; and the atleast one RF choke is preselected to have a high impedance at thepre-determined frequency so as not to significantly affect the resonanceof the loop.
 6. The distributed RF generator of claim 1, wherein eachactive component has a zero voltage switching (ZVS) configuration. 7.The distributed RF generator of claim 6, wherein: each ZVS configurationcomprises control circuitry that automatically performs ZVS based on asensed condition within the respective active component; and the sensedcondition within the respective active component comprises a sensed RFcurrent.
 8. The distributed RF generator of claim 7, wherein: a phasesynchronization of the at least one active component is performedautomatically in that all of the active component are phase synchronizedto the same RF current flowing in the loop; and each ZVS configurationcomprises a plurality of transistors configured as anelectrically-controllable switch that operates as ZVS amplifier.
 9. Thedistributed RF generator of claim 8, wherein each ZVS amplifier is aClass E amplifier.
 10. The distributed RF generator of claim 1, wherein:the DC power source is one of a plurality of direct current (DC) powersources; and the first end and the second end of theelectrically-conductive loop are adapted to be electrically coupled tothe plurality of DC power sources.
 11. A system for wireless powertransfer, comprising: a direct current (DC) power source; and adistributed radio frequency (RF) generator for wireless power transfer,comprising: an electrically-conductive loop having at least a first endand a second end that are adapted to be electrically coupled to the DCpower source, wherein the loop comprises a plurality of segments, eachof the plurality of segments comprising: a length of wire and at leastone active component, wherein the at least one active component has afirst terminal and a second terminal that are electrically coupled tothe loop, wherein: a DC voltage exists between the first terminal andthe second terminal; a DC current flows into the first terminal and outof the second terminal; an oscillating RF voltage is output across thefirst terminal and the second terminal; and the at least one activecomponent is synchronized in phase.
 12. The system of claim 11, wherein:the DC power source is one of a plurality of direct current (DC) powersources; and the first end and the second end of theelectrically-conductive loop are adapted to be electrically coupled tothe plurality of DC power sources.
 13. A method for wireless powertransfer, comprising: providing a direct current (DC) power source; andproviding a distributed radiofrequency (RF) generator comprising anelectrically-conductive loop having at least first and second ends thatare electrically coupled to the DC power source, wherein the loopcomprises a plurality of segments, each of the plurality of segmentscomprising: a length of wire and at least one active component, whereinthe at least one active component has a first terminal and a secondterminal that are electrically coupled to the loop, wherein: a DCvoltage exists between the first terminal and the second terminal; a DCcurrent flows into the first terminal and out of the second terminal; anoscillating RF voltage is output across the first terminal and thesecond terminal; and the at least one active component is synchronizedin phase.
 14. The method of claim 13, wherein the distributed RFgenerator further comprises: at least one passive sub-segment comprisinga length of wire and at least one passive component, the at least onepassive component comprising at least one capacitor; and at least oneactive sub-segment, the at least one active sub-segment comprising alength of wire and the at least one active component.
 15. The method ofclaim 13, wherein: the DC power source is one of a plurality of directcurrent (DC) power sources; and the first end and the second end of theelectrically-conductive loop are adapted to be electrically coupled tothe plurality of DC power sources.
 16. The method of claim 14, whereinthe at least one passive component comprises the at least one capacitorand at least one RF choke connected in parallel with the at least onecapacitor, wherein the at least one capacitor is preselected such thatthe loop is series resonant at a pre-determined frequency.
 17. Themethod of claim 16, wherein: the at least one RF choke includes aninductor; and the at least one RF choke is preselected to have a highimpedance at the pre-determined frequency so as not to significantlyaffect the resonance of the loop.
 18. The method of claim 13, whereineach active component has a zero voltage switching (ZVS) configuration.19. The method of claim 18, wherein: a phase synchronization of the atleast one active component is performed automatically in that all of theactive component are phase synchronized to the same RF current flowingin the loop; each ZVS configuration comprises a plurality of transistorsconfigured as an electrically-controllable switch that operates as ZVSamplifier; and each ZVS amplifier is a Class E amplifier.
 20. The systemof claim 11, wherein the distributed RF generator further comprises: atleast one passive sub-segment comprising a length of wire and at leastone passive component, the at least one passive component comprising atleast one capacitor; and at least one active sub-segment, the at leastone active sub-segment comprising a length of wire and the at least oneactive component.